NIR Spectroscopy of Exoplanet Host Stars

Recently I've gotten involved with the Sloan Digital Sky Survey APOGEE-2 survey, which uses twin spectrographs in the Northern and Southern Hemispheres to take 22,000 resolution H-band spectra of hundreds of stars at a time (as a multi-object, fiber fed spectrograph). Working in the near-infrared is particularly useful for measuring the compositions of cool, M dwarf stars, and my colleagues have been working to determine the best way to do this with APOGEE data. The automatic pipeline that produces abundances from the APOGEE spectra is not tuned to dwarf stars, but giant stars intead, so my collaborators and I are working to validate the parameters and abundances produced by the pipeline for dwarf stars; I want to use APOGEE-2 to observe TESS stars, of course! 

As a step in that direction, I worked with Robby Wilson from the University of Virginia to validate the effective temperatures and metallicities ([Fe/H] values) that the APOGEE pipeline produces for dwarf stars. Robby then took a look at the trends between the properties of small exoplanets and the metallicities of their host stars, and found that the short period planets (less than ~8.5 days) tend to orbit more metal-rich stars! We think this might be related to the dust sublimation radius being closer in around more metal-rich stars, but we need to work with more advanced models of protoplanetary disks to understand our observed trend better. 

From Wilson et al. (2018). Histogram of the host star metallicities of the long (red) and short (blue) period populations, split by P_crit=8.5 days. The combined distribution is shown in gray. The long period population peaks near solar metallicity while the short period population peaks above solar metallicity. The median host [Fe/H] is shown by the tick marks for the long (red) and short (blue) period populations.

I also participated a fun press release at the 2017 AAS meeting, using APOGEE abundances for two Kepler planet host stars to predict their interior compositions. 

Interior mineraology models for planets around two host stars detected by Kepler. The different phases and their relative fractions of the planet are estimated by knowing the radius of the planet and the composition of the star (from APOGEE). We hope to eventually be able to do this for hundreds more planets! (Note that we still need to validate these abundances, as I mention above!) Interior compositions calculated by Wendy Panero and Cayman Unterborn, beautiful grapic by Robin Dienel

Looking forward, I am very pleased to be a member of the previously-"After Sloan-IV", now officialy SDSS-IV team! You can read more about the survey, which will start in 2020, here. I helped write the science case for characterizing exoplanet host stars, and the education and public outreach plan. Specifically I'm involved with the Milky Way Mapper survey, and the chair of the TESS/planet host working group. Our goal is to get an H-band spectrum of every star in the Northern and Southern continuous viewing zones of TESS and JWST, as well as planet host stars discovered in the TESS full-frame images (FFIs). Consider joining SDSS-V and helping us characterize TESS stars! 

SDSS-V will be carried out in two hemispheres, at Apache Point Observatory (APO) and Las Campanas Observatory (LCO). Multi-object fiber spectroscopy will be obtained with two 2.5m telescopes, each feeding the near-infrared APOGEE and optical BOSS spectrographs, for the Milky Way Mapper and Black Hole Mapper surveys. The Local Volume Mapper will make use of integral-field spectroscopy, mostly at smaller telescopes at APO and LCO. (Image credit: M. Seibert (OCIS) & SDSS-V team)

This artist's impression shows a cutaway view of the parts of the Universe that SDSS-V will study. SDSS-V will study millions of stars to create a map of the entire Milky Way. Farther out, the survey will get the most detailed view yet of the largest nearby galaxies like Andromeda in the Northern hemisphere and the Large Magellanic Cloud in the Southern hemisphere. Even farther out, the survey will measure quasars, bright points of light powered by matter falling into giant black holes. Artist's Conception of SDSS-V by Robin Dienel/Carnegie Institution for Science/SDSS.